1. Technical Field
The present invention relates to a flexural mode resonator element, and further to a resonating device such as a resonator or an oscillator having a flexural mode resonator element housed in a package.
2. Related Art
For communication apparatuses such as mobile phones, information apparatuses such as personal computers, and other various electronic apparatuses, various types of resonating devices, such as resonators or oscillators, using a piezoelectric material or a non-piezoelectric material have been widely used. Especially for recent resonating devices, with the increased performance and reduced size of electronic apparatuses, a further reduction in size, highly accurate performance and high stability with the CI (Crystal Impedance) value being decreased are required.
For suppressing the CI value while reducing the size, a tuning-fork type resonator element which vibrates in a flexural mode generally adopts a structure in which a groove in a longitudinal direction is formed in front and back main surfaces of a vibration arm and an excitation electrode on the main surface side is formed on an inner surface of the groove. For eliminating variation in CI values between resonator elements while achieving size reduction by shortening a base in the tuning-fork type resonator element having such a structure, a technique has been known in which cutout portions are formed in both side portions of the base to prevent vibration leakage from the vibration arm to the base side (refer to JP-A-2002-261575, for example).
This tuning-fork type resonator element is bonded at the base on a mounting surface with a conductive adhesive or the like and supported thereon in a cantilever manner. Therefore, since an area for coating the adhesive is needed for the base, there is a limit to reduce the size. Moreover, since the distance from the vibration arm to the bonding position of the base is short, even when the cutout portions are disposed at the sides of the base, it may be impossible to sufficiently prevent the vibration leakage from the vibration arm.
A structure has been known in which a tuning-fork type resonator element is not fixedly bonded at a base, but supporting arms extending from the base parallel to vibration arms are disposed in a frame shape, and the tuning-fork type resonator element is fixedly supported at the supporting arms (refer to JP-A-2004-297198, for example). This tuning-fork type resonator element can achieve size reduction by reducing the dimension of the base in a longitudinal direction and can suppress vibration leakage by increasing the distance from the vibration arm to the fixed position of the supporting arm.
Also in the tuning-fork type resonator element which is fixedly supported at the supporting arms extending from the base parallel to the vibration arms as described above, it is proposed to dispose cutout portions at positions of the base nearer the vibration arms than the supporting arms (refer to JP-A-2006-148857, for example). With the cutout portions, an increase in CI value caused by the occurrence of vibration leakage from the vibration arm to the supporting arm via the base is prevented.
In the tuning-fork type resonator element disclosed in JP-A-2006-148857, each depth of the cutout portions is preferably set by reducing the width so that each bottom coincides with outer side edges of the vibration arms respectively adjacent to the cutout portions, for suppressing the vibration leakage. Further according to JP-A-2006-148857, the cutout portion is preferably formed from an end of the base to which the vibration arm is joined to a position exceeding the length corresponding to the arm width of the vibration arm. However, JP-A-2006-148857 does not refer to the effect that the length of the cutout portion has on the prevention of the vibration leakage, that is, on the reduction in CI value.
A flexural mode resonator element can be formed of a material having etching anisotropy, such as a quartz substrate. In this case, when a quartz substrate is wet etched to fabricate the outer shape of the resonator element, a protrusion, called a fin, is left on a side surface of the fabricated portion due to the orientation of a quartz crystal face because of the effect of anisotropic etching. Especially a tuning-fork type resonator element is more reduced in size and complicated in shape, and therefore, a fin having an unfavorable shape is generated at, for example, a coupling part of a supporting arm and a base, which may cause stress concentration to result in unstable vibration or may cause damage due to impact.